Fluid power systems are widely used in the industry for performing various types of work, such as generating high-velocity fluid jets. One important component of all fluid power systems is the pump through which the system fluid is pressurized and delivered. A variety of conventional mechanical components are used inside the pump to pressurize fluids and an electric motor, an engine, or another fluid power system typically provides the required energy. A pump is basically a device for converting kinetic energy from a prime mover to the potential energy stored in pressurized fluid, or for raising the potential energy from one fluid to another fluid by adding kinetic energy.
Conventional types of pumps have various names. The names often identify a mode of operation, method of pressurization or appearance of the pump. Common types of conventional pumps include centrifugal pumps, diaphragm pumps, roller pumps, vane pumps, bellows pumps, tubing pumps, screw pumps, piston pumps, crankshaft pumps, positive displacement pumps, and pressure intensifiers. At relatively low operating fluid pressures, there are many types of conventional pumps available and the design is often dictated by considerations such as fluid compatibility, cost and size. At relatively high operating fluid pressures, there are fewer types of conventional pumps available. At operating pressures above about 1,000 psi, there are only a few types of conventional pumps that can withstand the stresses involved and that are capable of producing the required pressures. At relatively high fluid pressures, such as above 10,000 psi, suitable conventional pumps are restricted to the so-called positive-displacement reciprocating pumps that involve constant speed moving pistons to move a fixed volume of fluid through a set of check valves and into the delivery line. Such conventional pumps may also be identified as axial-piston pumps, radial-piston pumps, and crankshaft pumps to denote the arrangement of the multiple pistons or plungers involved.
A conventional crankshaft pump is normally a multiple-piston pump that uses a crankshaft to impart linear movement to a set of pistons, such as those of known automotive engines. A conventional triplex pump has three cylinders or pistons; a quintuplex pump has five cylinders or pistons. Conventional crankshaft pumps are generally directly driven with electric motors or engines, normally at a rotating speed of about 500 rpm. A conventional crankshaft pump is shown in FIG. 1.
A conventional pressure intensifier is a piston pump that is driven with pressurized fluid, such as hydraulic fluid or another suitable working fluid, through a piston-plunger arrangement to raise the pressure of another fluid, the system fluid. The term pressure intensifier often implies that there are two separate fluids and fluid systems involved. The additional energy required is provided by a motor or engine of the working fluid system. Fluid pressure intensifiers are commonly used in generating relatively high-pressure waterjets at static pressures above about 40,000 psi. These intensifiers are often a double-acting type with two opposing plungers connected to a single power piston, which reciprocates within a cylinder as a result of pressurized hydraulic fluid alternatively entering the two sides of the power piston. Two piston position sensors and a pilot-operated 4-way hydraulic valve are conventionally used to regulate and control flow of a working fluid. The plungers, which often have a smaller cross-sectional area than the power piston, move the system fluid in and out of the high-pressure cylinders, through inlet and outlet check valves. An intensification ratio is defined as the area ratio of the power piston to the plunger, which determines a maximum pressure that the system fluid can attain inside a particular pressure intensifier. A conventional double-acting pressure intensifier is shown in FIG. 2.
The performance of a high-pressure pump is generally rated or defined by a peak-pressure capability, an efficiency, power characteristics, reliability, operating flexibility and cost. Key or primary components of high-pressure pumps include check valves, pistons, plungers, piston seals, and high-pressure cylinders. Because of the relatively high-frequency cyclic pressure pulsations and high internal stresses, these pumps parts are subjected to metal fatigue problems that result in premature fracture of metal parts. Reliability of these pump parts are very important to pump manufacturers and pump users.
Conventional crankshaft pumps are quite popular because of their direct-driven nature and rugged construction, and are used in many outdoor applications, such as irrigation and oil field operations. But they also have well-known shortcomings. Conventional crankshaft pumps generate relatively high vibrations due to the geometry of piston arrangement. For example, conventional triplex crankshaft pumps experience considerable output pressure pulsations due to their power distribution through only three cylinders. A quintuplex pump has improved pressure pulsation but is also bulkier and heavier because of the two additional cylinders. Crankshaft pumps are generally limited to a peak pressure of about 20,000 psi, due to metal-fatigue problems associated with the fluid manifold, which is often made of a monolithic block of stainless steel heavily ported and bored to accommodate the check valves and fluid passages. The complicated internal cavities of such fluid manifold have many stress-concentration sites that can develop fractures over a relatively short time, as a result of fluid pressure pulsations. Improved manifold design is a first step toward achieving increased operating pressures for crankshaft pumps.
Another shortcoming of conventional crankshaft pumps is the lack of operational flexibility, such as output pressure and flow control. External pressure-regulating and pressure-relief valves are required to provide some flexibility. Conventional valves suitable for very high fluid pressures are rare.
Hydraulically operated pressure intensifiers are well-suited for very high pressure applications, due in part to their smooth force transfer and good lubrication. They are the only pumps capable of reliably delivering fluids at pressures greater than about 40,000 psi. Unfortunately, conventional pressure intensifier systems are also more costly because of an extra hydraulic power unit. For example, a complete pressure intensifier system for waterjet applications will have a prime mover such as an electric motor or an engine, a hydraulic pump, a hydraulic reservoir or tank, a water-oil or air-oil heat exchanger or both, an oil filter, a 4-way solenoid-operated hydraulic valve, a double-acting intensifier equipped with power piston position sensors and circuit, an outlet pressure pulsation attenuator, a water inlet charge pump, water filters, support structure, tubing and hoses, and gauges and controls. A schematic diagram of a typical conventional fluid pressure intensifier system is shown in FIG. 3. One of such pressure intensifier systems is taught by U.S. Pat. No. 5,092,744.
The intensification ratio of intensifiers can be as low as about 2:1 or as high as about 20:1. For example, a conventional hydraulic pump capable of producing a 5,000 psi output pressure is commonly used in hydraulic power systems. Many of these pumps have advanced features, such as pressure compensation and output flow adjustment. When such a hydraulic power unit is used to power a pressure intensifier having a 20:1 intensification ratio, for example, an output system pressure of about 100,000 psi can be reliably produced. At present, system pressures considerably greater than about 100,000 psi are produced in such manner for several important yet uncommon applications.
Conventional pressure intensifiers operate relatively slowly; a reciprocating rate of 60 rpm is common. Relatively large intensifiers can be considerably slower because of larger and longer pistons. The relatively slow speed of conventional intensifiers is helpful from a metal-fatigue point of view. Because a double-acting intensifier has only two pistons, its output power continuity is very poor and pressure pulsations are very severe. Therefore, external pressure pulsation attenuators in the form of a dead-volume high-pressure accumulator are practically mandatory for use with pressure intensifiers. This situation also encourages the use of two or more double-acting intensifiers to form a network in order to dampen the output pressure fluctuations.
Multiple intensifiers can be phased together to produce a prescribed "firing order" by controlling the hydraulic fluid flowing in and out of the multiple intensifiers. The aim is to produce as even as possible a power output from the reciprocating motion of all the pistons involved. Electrical drives, mechanical drives or a combination of both are conventionally used to yield such phased operations, but with only partial success. Multiple intensifiers can significantly increase the cost of the system equipment. The high cost of pressure intensifier system equipment is a cause for the current limited growth of waterjet technology.
Another shortcoming of conventional pressure intensifiers is their inflexible power capability. Once constructed, a pressure intensifier has a fixed maximum power output. If greater power output is desired, a physically larger pressure intensifier must be constructed, or another intensifier of the same type must be added to the system. Relatively large intensifiers have larger and longer pistons and therefore must operate at a slower speed, thus resulting in a longer dead moment during the reciprocating movement and aggravating the pressure pulsation problem.
A pressure intensifier of moderate power output is quite bulky and heavy. For example, a conventional 50 hp pressure intensifier may have a 5 inch diameter power piston and can be 40 inches long. A conventional double-acting 50 hp pressure intensifier may have two massive stainless steel end blocks, such as with 8 inches by 8 inches by 4 inches dimensions, and two pressure cylinders, such as with 4 inches by 10 inches dimensions. The amount of expensive materials involved in each pressure intensifier is quite substantial and yet a 50 hp power output is quite modest for waterjetting applications. When one major component of a conventional pressure intensifier fails, such as a cylinder, it is simply discarded.
In view of the current status of high-pressure pumps available for waterjet and other applications, there is quite a demand for significant improvement in the pump design, so that the performance can be improved and the cost reduced. This invention is aimed at accomplishing at least these two basic objectives.